Navy ships and commercial vessels operate in virtually all of the world's waterways. For the purpose of waste management, these ships can be viewed as small floating cities. Solid wastes surveys conducted aboard U.S. Navy ships indicate that approximately 3.0 pounds (1.4 kilograms) of solid wastes are generated per person per day while at sea. Thus, the people on a typical aircraft carrier generate over 9.9 tons (9 tonnes) of solid wastes per day. Overall, thousands of tons of solid wastes are generated throughout the world's waterways. Presently, the vast majority of these wastes are dumped into the sea, creating significant environmental problems.
In 1987, the United States Senate unanimously ratified an international law which prohibits the discharge of solid wastes into certain "special areas" of the world's oceans. The application of this law extends to all public vessels, including U.S. Navy ships. A congressional provision in 1993 established a deadline of Jan. 1, 2000, for the U.S. Navy surface fleet to meet this requirement.
Presently, the U.S. Navy needs a waste treatment facility which can be installed on board ships. The facility must be lightweight, compact, and modular, so that it can be transported in and out of ships without the need to modify the ship's basic design. Furthermore, the facility must offer high energy efficiency and quick startup or shut-down potential. The technology, disclosed herein, meets all the needs of the U.S. Navy. However, the disclosed technology can also be used for any application where weight and size must be kept to a minimum, such as in transportable or mobile waste destruction systems.
One of the most versatile technologies for the destruction of solid wastes is thermal treatment. Thermal treatment can be accomplished either by incineration or by pyrolysis.
In incineration, the solid wastes are essentially burned in an oxygen rich environment. The combustible constituents of the wastes, such as paper, plastics and other organic compounds, are used as the fuel, with additional hydrocarbon fuels added as needed to maintain a sufficiently hot flame. Typically, incineration is characterized by very large furnaces, incomplete combustion, the generation of polluting emissions, such as dioxins, and the generation of ashes. As such, conventional incineration is not a practical technology for use as a light, compact facility that must generate no detectable emissions.
A number of inventors have described incineration technologies which are specifically aimed at making the systems more compact and efficient. U.S. Pat. No. 5,353,720 describes an incineration system which uses pressurized oxygen and hydrogen, instead of the conventional air and either gas or oil, to create a flame temperature within the chamber of at least 4,000.degree. F. (2,250.degree. C.). The burning of oxygen and hydrogen significantly reduces the amount of gas needed and therefore, the size of the unit. Furthermore, by burning at extremely high temperature, the polluting emissions in the off-gas are dramatically reduced. However, one may expect that this type of operation is extremely dangerous and the risk of explosions may hinder the commercial use of this technology. Additionally, large amounts of hydrogen and oxygen must be available for operating this type of furnace.
Other incineration based technologies are described in U.S. Pat. Nos. 4,627,365 and 4,579,067, respectively. U.S. Pat. No. 4,627,365 describes a portable incineration facility which is installed onto the truck that also collects municipal garbage. This type of incineration is designed to process very small quantities of solid wastes and it is doubtful that this approach will offer satisfactory waste destruction and clean off-gas emissions. U.S. Pat. No. 4,579,067 describes an incinerator design which offers higher combustion temperatures in an effort to reduce the emission of polluting compounds. All incineration furnaces produce fly ash which is both toxic and difficult to handle and which must either be land-filled or further processed.
Other incineration type furnaces include those described in U.S. Pat. Nos. 4,479,443 and 4,848,250, respectively. Both of these technologies use incineration coupled with an additional energy source in an effort to improve the performance of the furnace. U.S. Pat. No. 4,479,443 describes a furnace which uses both a plasma and a combustion flame to destroy PCB's. The plasma is used to increase the combustion temperature and, thus, increase the destruction efficiency of the PCB's to greater than 99.99%. In U.S. Pat. No. 4,848,250, an induction coil is used to maintain a molten metal bath into which the solid wastes are dropped and treated. This system, which combines induction and incineration, offers the ability to both combust the wastes at higher temperatures, thus, reducing the polluting emissions in the off-gas, and vitrify the fly ash.
In pyrolysis, the waste is exposed to heat in excess of 1,000.degree. F. (540.degree. C.) in an oxygen deficient environment. Electrical energy, such as that transferred by plasma jets, arcs and induction furnaces, are used as the sources of heat. The water in the wastes vaporizes and the organics dissociate to form simpler volatile compounds, such as H.sub.2 CO, CO.sub.2, and C.sub.2 H.sub.2. The remaining materials, mostly ash, glass and metals, can be heated further to temperatures in the order of 3,500.degree. F. (1,930.degree. C.) to melt and vitrify into a slag, which has been shown to be extremely stable and to possess no environmental threats. In fact, vitrified slag is so stable that it is considered the most accepted method of storing even the most dangerous materials, such as nuclear wastes. The off-gas, containing all the dissociated volatile compounds must be treated further to prevent any toxic emissions. Typically, this treatment involves combustion for 2 seconds at a minimum of 2,000.degree. F. (1,095.degree. C.). Air or oxygen are used for combustion. A hydrocarbon fuel is used to ensure that the temperature is maintained above 2,000.degree. F. (1,095.degree. C.).
A number of pyrolysis furnaces have been designed and patented over the past two decades. In one design, developed by Retech Inc. (U.S. Pat. No. 4,770,109), a plasma torch is used to supply the energy for pyrolysis. Air is used as the plasma forming gas. The entire furnace is rotated, causing the molten wastes to spread out over the surface of the furnace. By spreading out the molten wastes, the surface area available for treatment is increased and Retech claims that the quality of the treatment is enhanced. The vitrified slag is tapped through a hole located at the center of the rotating furnace by reducing the turning velocity, and the associated centrifugal force, and allowing the slag to flow into the hole.
Another plasma pyrolysis furnace is described in U.S. Pat. No. 5,143,000 awarded to Plasma Energy Corporation (PEC). In the PEC furnace the solid wastes are fed into the furnace forming a column. The plasma torch is inserted into the lower portion of the column having the plasma emitting end surrounded by the wastes. The height of the column is maintained at a level above the plasma torch. The energy of the plasma is used to dissociate the organic compounds in the mixed wastes and to melt the ash and the metals into a slag. The volatile organics leave through the top of the furnace, while the slag is removed from the bottom. In the PEC furnace both the plasma torch and the furnace wall are fixed and any movement is a result of momentum transferred to the wastes by the plasma jet. However, a movement for the torch, such as the one described for a graphite arc electrode in U.S. Pat. No. 4,982,410, presumably could be introduced, if required.
U.S. Pat. No. 5,280,757 also describes a plasma pyrolysis furnace similar to the Retech furnace and the PEC technologies but does not offer details of the design. This patent claims a process in which a plasma arc is used inside a refractory lined furnace to heat Municipal Solid Wastes (MSW) to temperatures in excess of 2,010.degree. F. (1,100.degree. C.). The entry of air into the furnace is minimized in an apparent effort to produce an off-gas with a higher heating value. In pyrolysis, the off-gas contains enough hydrogen and hydrocarbons to burn it and recover approximately 250 Btu/ft.sup.3 (9,315 kJ/m.sup.3) of gas.
All the furnaces described in the above mentioned patents, whether they use incineration or pyrolysis, use conventional refractory liners to cover the inner surface of the furnace and prevent both excessive heat loss and damage to the structural walls. Furthermore, these designs must always be accompanied by a Secondary Combustion Chamber (SCC) for the treatment of the off-gas. Typically, the inner surface of the SCC must also be covered with conventional refractory liners. Conventional liners, even those made from advanced materials such as those developed by Norton Company and described in U.S. Pat. No. 4,823,359, add tremendously to the weight of the furnace and the SCC. For example, plasma furnaces, such as those described in U.S. Pat. Nos. 4,770,109 and 5,143,000, respectively, operating at 500 kW and able to treat about 700 lb/hr (320 kg/hr) of solid wastes, combined with a suitable SCC system, would need enough refractory to cover approximately 200 ft.sup.2 (19 m.sup.2) of furnace wall. If energy losses as high as 20% are acceptable, the thickness of the conventional zirconia refractory wall would be approximately 12 inches (30 cm), and the furnace and SCC would require almost 40 tons (44 tonnes) of refractory. If lower losses are desired, the refractory wall would have to be thicker and heavier. Another problem with the use of conventional refractory lined furnaces is the long time required for heat-up and cool-down of the furnace during start-up and shut-down.
The amount of refractory needed can be reduced significantly if the furnace operation does not require the use of a SCC. In U.S. Pat. No. 4,644,877, a spray ring is used to quench and clean the off-gas created from the plasma dissociation of wastes. In U.S. Pat. No. 5,579,705, the off-gas from the plasma furnace is treated by flowing co-currently with the slag generated in the furnace. In both of these technologies, the treatment of the off-gas is aimed at the elimination of a SCC system. However, neither of the technologies described in these patents is considered to offer adequate treatment, and in most applications, U.S. law requires the use of a SCC system.
The best way to avoid the added bulk and weight of the furnace, contributed by the refractory lining, is to use a lighter energy management system. The development of an improved refractory wall has been the focus of many technologies. For example, U.S. Pat. No. 4,802,425 describes a refractory lining which consists of alternating strips of two fibrous materials. The first fibrous material is chosen for its shrinkage or corrosion resistance while the second material is chosen for its superior mechanical strength. While this type of design may offer improved performance in highly corrosive environments, it is not obvious that it will significantly reduce the bulk or the weight of the lining.
A structure for insulating furnaces, which does not use conventional refractory materials, is described in U.S. Pat. No. 4,398,474. This structure consists of a shell, enclosing a series of refractory, substantially parallel, spaced-apart sheets, one or more of which is reflective. The insulating wall is designed to work by reflecting part of the radiation from the hot surface back to the heat source, thereby reducing heat losses. This structure appears to be based on a theoretical study and leaves many unanswered practical questions. For example, the patent does not specify any materials that may be used to obtain the desired properties. In fact, the described structure seems more suitable for use in relatively low temperatures (about 1,000.degree. F. or 540.degree. C.) and for containing the heat generated by a clean energy source. To the knowledge of the present inventors, no such insulating structure has ever been used for wastes treating furnaces.
An issue which must be considered in designing a lightweight furnace wall for use in waste-treating furnaces relates to the large variations in the composition of the feed materials. Some wastes contain large amounts of organics which, when combusted, release tremendous amounts of energy. Other waste feeds contain little or no organics and, therefore, their treatment results in minimum energy production. Since the organic content of the feed can change constantly, the energy input to the furnace wall changes, and the cooling rate of the wall must vary accordingly in order to maintain a constant wall temperature. This issue is particularly important for walls designed to heat up and cool down rapidly and which, therefore, must store minimum energy, as compared to conventional refractory walls that store a huge amount of energy and can stay hot despite temporary reductions in the heat released within the furnace.